TECHBRIEF Bond Behavior of 

Reinforcing Steel in 

Ultra-High Performance 

Concrete

FHWA Publication No.: FHWA-HRT-14-089

FHWA Contact: Ben Graybeal, HRDI-40, 202-493-3122, 

benjamin.graybeal@dot.gov

This document is a technical summary of the Federal 

Highway Administration (FHWA) report Bond Behavior 

of Reinforcing Steel in Ultra-High Performance Concrete 

(FHWA-HRT-14-090), available online at http://www.fhwa.

dot.gov/publications/research/infrastructure/structures/

bridge/14090/index.cfm.

Objective

Ultra-high performance concrete (UHPC) has garnered 

interest from the highway infrastructure community for 

its greatly enhanced mechanical and durability properties. 

The objective of this research is to extensively evaluate 

the factors that affect bond strength between deformed 

reinforcing bar and UHPC, and to facilitate the development of design guidelines for using field-cast UHPC in 

innovative connection details. The results of the research 

effort and the design recommendations for reinforcing bar 

embedded in UHPC are provided in this TechBrief.

Introduction

The use of accelerated bridge construction (ABC) techniques continues to grow as owners across the country 

look for construction solutions that reduce impacts on 

the users of the infrastructure. In ABC construction, one 

common technique used is prefabricated bridge elements 

and systems (PBES). In this practice, bridge elements are 

prefabricated offsite and then assembled and connected 

onsite during an expedited construction timeframe. 

The use of prefabricated bridge elements necessitates 

the use of field-applied connections between these 

Research, Development, and 

Technology

Turner-Fairbank Highway 

Research Center

6300 Georgetown Pike

McLean, VA 22101-2296

www.fhwa.dot.gov/research
2

elements. Field-cast concrete or other 

cementitious material connections have 

been deployed for decades by State transportation departments. However, decades 

of experience has led to the recognition that 

the field-cast connections often prove to be 

susceptible to degradation that can lead 

to substandard performance of the overall 

bridge system.

UHPC is an advanced construction material 

that affords new opportunities to envision 

the future of the highway infrastructure. 

Since 2000, when UHPC became commercially available in the United States, a series 

of research projects has demonstrated 

the capabilities of the material. A handful of State transportation departments 

have deployed UHPC components within 

their infrastructure, and many more are 

actively considering the use of UHPC. Many 

State transportation departments, bridge 

design firms, and construction firms have 

expressed their interest in using UHPC in 

bridge construction, especially for field-cast 

connections deployed in the construction of 

PBES structures. As of late 2013, 34 bridges 

in the United States have been constructed 

using field-cast UHPC connections.(1)

As opposed to conventional grouted connections, which frequently contain complex 

reinforcement configurations, UHPC connections often involve much simpler reinforcement configurations such as the lap 

splicing of straight lengths of reinforcement. 

A few specific connection details, such as 

those discussed in Behavior of Field-Cast 

Ultra-High Performance Concrete Bridge 

Deck Connections Under Cyclic and Static 

Structural Loading and Development of a 

Field-Cast Ultra-High Performance Concrete 

Composite Connection Detail for Precast 

Concrete Bridge Decks, have been rigorously 

tested at service and ultimate performance 

limits.(2,3) The advanced material properties 

of UHPC provide an opportunity to develop 

simple and robust connection systems for 

prefabricated bridge elements.

The research discussed in this document 

focuses on the assessment of bond performance of deformed reinforcing bar in UHPC. 

This is an ongoing research program at the 

FHWA Turner-Fairbank Highway Research 

Center as part of a larger effort focused on 

developing innovative connection details 

for prefabricated bridge components.

UHPC

Advances in the science of concrete materials have led to the development of a new 

class of advanced cementitious materials, 

namely UHPC. These concretes tend to 

contain high cementitious materials contents and very low water-to-cementitious 

materials ratios, and to exhibit high compressive and tensile strengths. The discrete 

steel fiber reinforcement included in UHPC 

allows the concrete to maintain tensile 

capacity beyond cracking of the cementitious matrix. UHPC has been defined as 

follows:

UHPC is a cementitious composite 

material composed of an optimized 

gradation of granular constituents, a 

water-to-cementitious materials ratio 

less than 0.25, and a high percentage 

of discontinuous internal fiber reinforcement. The mechanical properties 

of UHPC include compressive strength 

greater than 21.7 ksi (150 MPa) and 

sustained post-cracking tensile strength 

greater than 0.72 ksi (5 MPa). UHPC 

has a discontinuous pore structure 

that reduces liquid ingress, significantly 

enhancing durability compared to conventional concrete.(1,4)
3

TESTING PROGRAM

Direct tension pullout tests, with a novel 

test specimen design and associated loading apparatus, were conducted in this study. 

The test setup was developed to mimic 

the tension-tension lap splice configuration that may be encountered in a fielddeployed connection system. As shown in 

figure 1, the pullout tests specimens were 

UHPC strips cast on top of precast concrete 

slabs. The No. 8 bars extended 8 inches 

(20.3 cm) from the precast concrete slab. 

UHPC strips were cast on top of the precast 

slab with the No. 8 bars in the center of the 

strips. Each testing bar was situated so as 

to be embedded into the UHPC strip and 

located between two of the No. 8 bars.

In figure 1, notations are assigned to represent dimension parameters, including cso for 

the clear side cover, 2csi for the clear spacing 

between the testing bar and the extended 

No. 8 bars, ld for the embedment length of 

testing bar measured from the top surface 

of the UHPC strip to the end of the testing 

bar, and ls for the lap splice length measured from the end of the testing bar to the 

end of extended No. 8 bars. The notations 

of csi, cso, ld, and ls are adopted from ACI 408 

R-03, Bond and Development of Straight 

Reinforcing Bars in Tension.(5)

The pullout tests were conducted using the 

fixture showing in figure 2. A hydraulic jack 

was placed on a steel chair, and the steel 

chair stands on the precast slab. When a 

pullout force is applied, the fixture reacts 

against the precast slab. With such a setup, 

the reinforcing bars being tested as well as 

the extended No. 8 bars are both placed 

in tension. The UHPC surrounding these 

bars transfers the loads between them. This 

test setup simulates structural configurations wherein lap spliced reinforcement is 

loaded in tension.

Over two hundred pullout tests were completed, and the following parameters were 

evaluated for their effect on the bond performance:

• Embedment length, ld.

• Concrete side cover, cso.

• Bar spacing, csi.

• Concrete compressive strength, f’c.

Figure 1. Overall configuration of test specimens.
4

• Reinforcing bar type.

• Reinforcing bar size.

• Reinforcing bar yielding strength.

• Casting orientation.

Test Results

Embedment Length

The effect of embedment length on bond 

strength was evaluated. All specimens analyzed in this section used A1035 No. 5 bar 

and had a center-to-center spacing between 

the testing bar and the nearest No. 8 bar of 

4 inches (10 cm). Figure 3 shows the test 

results of bar stress at bond failure versus 

the embedment length. The specimens 

shown in figure 3 were divided into four 

groups, and in each group, the specimens 

had the same design except for the varied 

embedment length. For example, all specimens in group “cso = 2db, 1D” have a side 

cover of 2db, and the tests were conducted 

1 day after casting when the UHPC 

compressive strength averaged 13.7 ksi 

(90 MPa); the only variable among the 

specimens is the embedment length. 

Similarly, specimens in each of the other 

groups have the same design except for 

embedment length, and the way each group 

is named follows the same rule as in the 

example, showing the information of the 

side cover in terms of reinforcing bar diameter and testing age.

As shown in figure 3, increasing the embedment length of a reinforcing bar increases 

the bond strength. The relationship between the bar stress at bond failure and the 

bonded length is nearly linear, similar to 

that observed in normal strength concrete 

(ACI 408 R-03).(5) The linear relationship 

between bond force and the bonded 

length in normal-strength concrete is often 

explained based on the assumption that all 

lugs bear against concrete at the ultimate 

stage and help in resisting the applied axial 

force, therefore at ultimate, the bond stress 

distribution is nearly uniform. However, the 

bond stress distribution in high-strength 

concrete, with compressive strength over 

Figure 2. Loading setup.
5

13 ksi (90 MPa) and without fiber reinforcement, was found not to be uniform in a 

study conducted by Azizinamini et al.(6) The 

same researchers also noted that for highstrength concrete, the increase in bearing 

capacity is more than the increase in tensile strength, which in turn would prevent 

crushing of the concrete in the vicinity of 

each lug to the extent that would otherwise 

take place in normal-strength concrete. In 

other words, the high-strength concrete 

would crack before crushing due to the 

less than proportional increase in tensile 

strength compared with bearing capacity. 

All lugs may not participate in resisting 

applied axial load before the concrete 

cracks, and the first few lugs contribute 

the most. The linear relationship observed 

in this study implies that the behavior 

attributed to conventional high-strength 

concretes by Azizinamini et al. may not be 

present in UHPC, potentially due to the 

enhanced pre- and post-cracking tensile 

response of the UHPC.

Concrete Side Cover and Bar Spacing

Concrete cover and bar spacing play important roles in bond strength. In general, 

for bond failure involving splitting of the 

concrete, the nature of the splitting failure 

depends on whether the concrete cover, 

cso, is smaller than csi, which is half of the 

clear spacing to adjacent bar. The demonstration of cso and csi is presented in 

figure 4. When cso is smaller than csi, the 

splitting crack occurs through the cover to 

the free surface, as shown in part (a) of 

figure 4. When csi is smaller than cso, the 

splitting crack forms between the reinforcing bars as shown in part (b) of figure 4. 

However, the results for reinforcing bar 

embedded in UHPC reveal that when csi 

is a constant value and smaller than cso, 

the bond strength still increases as the 

Figure 3. Effect of embedment length: fs,max versus embedment length ld.
6

side cover cso increases, instead of being 

controlled by csi. The models that used 

bar spacing and bar cover to predict bond 

strength in conventional concrete may 

need to be reevaluated in consideration 

of the added crack propagation resistance 

provided by fiber reinforcement in UHPC. 

The research also found that the specimens 

with contact lap splices exhibited lower 

bond strengths than specimens with noncontact lap splices, probably due to the fact 

that the tight spacing between bars limits the ability of the fiber reinforcement to 

locally enhance the mechanical resistance 

of the UHPC. However, when the specimen 

was situated far away from each adjacent 

bar, the adjacent bars do not contribute to 

the bond strength, and the performance 

becomes a function of the mechanical properties of the UHPC. These specimens exhibited decreased bond strength as compared 

with specimens with the closer non-contact 

lap splice configuration.

Compressive Strength

Traditionally, the effect of concrete properties on bond strength is represented by 

the square root of the compressive strength 

( ), which is related to tensile strength of 

the concrete. An increase in the concrete 

compressive strength increases the bond 

strength. The same is true for UHPC that an 

increase in the UHPC compressive strength 

increases the bond strength. However, the 

effect of UHPC properties on bond strength 

cannot be effectively represented by the 

compressive strength (f’c) or the square root 

of its compressive strength ( ). Other 

UHPC mechanical properties, particularly 

those relevant to the post-cracking tensile 

behavior of UHPC, may be more appropriate for evaluating the bond strength of 

reinforcing bar in UHPC.

Bar Size

The effect of reinforcing bar size on bond 

strength is evaluated using A1035 No. 4, 

No. 5, and No. 7 bars. As shown in figure 6, 

the bond strength for No. 4 and No. 5 bars 

are compared in set 1, and the No. 7 and 

No. 5 bars are compared in set 2. In each 

set, all the specimens with different sizes 

of bar were designed to have the same 

side cover, bar spacing, and UHPC compressive strength; the embedment length 

varied. Specimens in each set can be either 

directly compared among those with the 

same embedment length or compared as 

a whole by comparing the bond strength, 

which included the embedment length in 

the calculation. The bond strength is calculated by dividing the bond force at failure by 

the overall contact area using the equation 

shown in figure 5.

Figure 4. Bond splitting cracks.

Figure 5. Bond strength calculation.
7

where, fs,max is the bar stress at bond failure, 

db is the bar diameter, and ld is the embedment length. In addition to the average values of bond strength, figure 6 also shows 

the maximum and minimum values for 

the included specimens and the number of 

tests conducted. As shown in figure 6, No. 4 

bars have slightly higher average bond 

strength than the corresponding No. 5 bars 

in the first set of comparison, and No. 5 bars 

have higher average bond strength than the 

No. 7 bars in the second set of comparison. 

The tests results indicated that bar diameter 

does influence bond strength and that smaller 

bars exhibit comparatively larger resistance.

Bar Type and Yield Strength

Three types of reinforcing bar are evaluated 

in this study, specifically A615 uncoated 

Grade 60 bar, A615 epoxy coated Grade 

60 bar, and A1035 uncoated high strength 

Grade 120 bar. The specimens were grouped 

into six sets based on different design 

details, and in each set, all the specimens 

have the same design except for the bar 

type. The average bar stress at bond failure 

for each type of bar in each set is presented 

in figure 7. As shown, for specimens with 

ultimate bar stress at bond failure below or 

close to the yield strength of the uncoated 

Grade 60 bar (sets 1 and 2), the uncoated 

Grade 120 and Grade 60 bar had similar 

bond strength. When the bar stress at bond 

failure was greater than the yield strength 

of the uncoated Grade 60 bar, the uncoated 

Grade 120 bar had higher ultimate bar stress 

than the corresponding uncoated Grade 60 

bar. In all cases, the epoxy-coated bar had 

lower ultimate bar stress than the corresponding uncoated bar.

Figure 6. Bond strength versus bar size.
8

Casting Orientation

The UHPC tested in this study contains 

2 percent (by volume) steel fibers. Casting 

technique can influence the dispersion and 

orientation of the fiber reinforcement. In 

this study, the UHPC strips were prepared 

using plywood forms, and two cast orientations were compared, as shown in figure 8. 

The first orientation involved casting the 

specimen on its side as shown in parts (a) 

and (b) of figure 8; the second orientation 

involved casting the specimen upright as 

shown in parts (c) and (d) of figure 8, where 

the slab was placed with a small slope of 

approximately 1.5 degrees to facilitate the 

flow of the UHPC. For both orientations, the 

UHPC was first poured in from one end and

allowed to flow until the forms were mostly 

filled. Thereafter, the UHPC was poured in 

from the middle locations. The study found 

that the orientation of the casting did not 

have an obvious effect on bond behavior, 

and, for the purpose of easy construction 

and casting, the upright orientation was 

used in most of the tests. However, it was 

noted that the very end specimens that are 

closest to the casting point (where the large 

majority of the UHPC is poured into the 

formwork) consistently displayed slightly 

lower bond strength than specimens at the 

other positions. This behavior is probably 

related to the variation of fiber distribution 

inside the UHPC.

Figure 7. Average bar stress at bond failure for different types of reinforcing bar.
9

Figure 8. UHPC strip casting setup and orientation.

Conclusions

The bond strength of mild steel reinforcing 

bars in a commonly available UHPC was 

evaluated in this study. It was found that the 

bond behavior of deformed bars in UHPC 

is different from that in traditional concrete 

in many aspects. The following conclusions 

were developed based on the results of the 

tests completed in the study.

• Increasing the embedment length of 

the reinforcing bar increases bond 

strength.

• The relationship between the bond 

strength and the bonded length for 

bar embedded in UHPC is nearly 

linear, indicating that UHPC exhibits 

enhanced performance as compared 

with traditional high-strength concrete. 

• Bond strength increases as the side 

cover increases.

• Non-contact lap splice specimens 

exhibit higher bond strength than contact lap splice specimens, likely due to 

the fact that the tight spacing in contact lap splice limits the ability of the 

fiber reinforcement to locally enhance 

the mechanical resistance of the UHPC.

• When the bar clear spacing is so large 

that the induced diagonal cracks from 

the pullout force will not intersect with 

the adjacent bars, the adjacent bar will 
10

not help stop the propagation of the 

diagonal cracks and the bond strength 

becomes a function of the mechanical 

properties of the UHPC.

• Models that used bar spacing and 

side cover to predict reinforcing 

bar bond strength in conventional 

concrete may need to be reevaluated 

in consideration of the added crack 

propagation resistance provided by the 

fiber reinforcement in UHPC.

• An increase on the compressive 

strength of the UHPC results in an 

increased bond strength.

• The effect of UHPC properties on 

bond strength cannot be effectively 

represented by the compressive 

strength (f’c) or the square root of its 

compressive strength ( ). Other 

UHPC mechanical properties, particularly those relevant to the postcracking tensile behavior of UHPC, 

may be more appropriate for evaluating the bond strength of reinforcing 

bar in UHPC.

• For bars with larger diameter, the bond 

strength decreases.

• Bars that yield before bond failure 

have less ultimate bond strength than 

similarly configured high-strength bars 

that do not yield before bond failure.

• The epoxy-coated bars have lower 

bond strength than similarly configured 

uncoated bars.

Recommendations

One of the main goals of the research is to 

develop design recommendations for reinforcing steel embedded in UHPC, thus providing guidance for designers using reinforced UHPC in innovative applications. This 

study focused on a widely available UHPC 

product containing 2 percent steel fiber (by 

volume). Reinforcing bar sizes ranging from 

No. 4 to No. 8 and bar types including A615 

Grade 60 uncoated and epoxy-coated bar 

and A1035 Grade 120 bar were included in 

the study.

Deformed bar embedded in UHPC can attain 

the lesser of the bar yield strength or 75 ksi 

(517 MPa) at bond failure when the following conditions are met:

• Bar size from No. 4 to No. 8.

• Uncoated or epoxy-coated bar.

• Minimum embedment length of 8db.

• Minimum side cover of 3db.

• Bar clear spacing between 2db and ls.

• Minimum UHPC compressive strength 

of 13.5 ksi (93 MPa).

For lap splice reinforcement configurations, 

a minimum lap splice length of 75 percent 

of the embedment length is suggested, 

which is the range into which most of the 

tests in this study fell. Note that db is the bar 

diameter and ls is the lap splice length.

For situations in which the above conditions 

are met except that the minimum side cover 

is between 2db and 3db, the minimum embedment length should be increased to 10db.

Refinements of the recommended design 

can be made for specific applications. For 

example, if a larger side cover is provided, 

and/or UHPC has gained higher compressive 

strength, an embedment length reduction 
11

may be possible. The supporting information can be found in the associated report, 

Bond Behavior of Reinforcing Bar in UltraHigh Performance Concrete, available 

through the National Technical Information 

Service at www.ntis.gov.

References

1. FHWA. (2014). “Ultra-High Performance 

Concrete,” Federal Highway Administration Research and Development. 

Accessed at: https://www.fhwa.dot.gov/

research/resources/uhpc/.

2. Graybeal, B. (2010). Behavior of FieldCast Ultra-High Performance Concrete 

Bridge Deck Connections Under Cyclic 

and Static Structural Loading, Report 

No. HRT-11-022, Federal Highway 

Administration, McLean, VA.

3. Graybeal, B. (2012). Development of 

a Field-Cast Ultra-High Performance 

Concrete Composite Connection Detail 

for Precast Concrete Bridge Decks, 

Report No. HRT-12-042, Federal Highway 

Administration, McLean, VA.

4. Russell, H.G., and Graybeal, B. (2013). 

Ultra-High Performance Concrete: A 

State-of-the-Art Report for the Bridge 

Community, Report No. HRT-13-60, 

Federal Highway Administration, 

McLean, VA.

5. ACI Committee 408. (2003). Bond and 

Development of Straight Reinforcing 

Bars in Tension (ACI 408R-03), American 

Concrete Institute, Farmington Hills, MI.

6. Azizinamini, A., Stark, M., Toller, J.J., and 

Ghosh, S.K. (1993). “Bond Performance 

of Reinforcing Bars Embedded in HighStrength Concrete,” ACI Structural 

Journal, Vol. 90, No. 5, pp. 554–561.
12

NOVEMBER 2014 FHWA-HRT-14-089

HRDI-40/11-14(300)E

Researchers—This study was led by Ben Graybeal at FHWA’s Turner-Fairbank Highway Research

Center. It was conducted by Jiqiu Yuan of PSI, Inc., through laboratory support contract DTFH61-

10-D-00017. Additional information can be obtained by contacting Ben at (202) 493-3122 or in 

the FHWA Office of Infrastructure Research and Development located at 6300 Georgetown Pike, 

McLean, VA 22101. 

Distribution—This TechBrief is being distributed according to a standard distribution. 

Direct distribution is being made to the Divisions and Resource Center.

Availability—This TechBrief may be obtained from the FHWA Product Distribution Center 

by email to report.center@dot.gov, fax to (814) 239-2156, phone to (814) 239-1160, or online at 

http://www.fhwa.dot.gov/research. 

Key Words—Ultra-high performance concrete, UHPC, fiber reinforced concrete, bond strength, 

deformed reinforcing bar, prefabricated bridge elements and systems, PBES.

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